Extrusion is a well known process in which molten metal or some meltable substance such as plastic is introduced into the shot sleeve of a die casting machine or an injection molding device. The substance to be extruded is then forced by the application of pressure into the die cavity of a mold. Pressure is exerted by activating either a reversibly movable plunger or a screw extruder within the shot sleeve. Turning the screw extruder or moving the plunger forward through the shot sleeve will force the extrudant into the die cavity.
The process of forcing molten metal or some meltable substance into a die cavity produces friction between the extrudate and the metal surfaces within the shot sleeve. These metal surfaces include the inside barrel of the shot sleeve and, if used, the threads of a screw feeder. The amount of friction will depend upon the material being injected, the temperature inside the shot sleeve, and the amount of pressure being applied.
It is desirable that the amount of friction be controlled. In the context of metal die casting, the complete elimination of friction is desired. Friction increases the amount of power required to drive the motor which rotates the screw feeder or activates the plunger. It also acts as a source of physical wear as extrudate is forced by pressure into the die cavity.
In other applications, the presence of some friction is desirable. An example occurs during plastic injection molding where pellets of polymeric material are mixed and melted through the extrusion process. A certain amount of friction is needed in order to cause melting of the pellets as they move through the stationary barrel, or shot sleeve.
Whether the application involves metal die casting, plastic injection molding, or some other casting process, a way of controlling the level of friction inside the shot sleeve during the extrusion process can be achieved by introducing a lubricant. Prior art reveals efforts which have been made by others to introduce either liquid or solid lubricating additives. U.S. Pat. No. 4,877,568 discloses a process by which a liquid lubricant is mixed with pellets of polymeric material during injection molding by forcing the lubricant through a canal or "root" formed internal to the shaft of a screw feeder. The lubricant is able to travel through the screw shaft and then empty into the shot sleeve through an orifice within the screw which fluidly connects the shot chamber with the root.
U.S. Pat. No. 5,076,339 discloses the use of a solid lubricant in the form of a stick. The lubricant stick is manually inserted into a pour hole formed within the shot chamber proximal to the input end of the shot sleeve. The lubricating stick melts upon contact with the shot sleeve. This method, of course, requires the user to operate the die casting machine without lubricant until it becomes heated, and then manually insert the solid lubricant. This, in turn, subjects the operator to the risk of burns to his or her fingers.
Neither U.S. Pat. No. 4,877,568 nor U.S. Pat. No. 5,076,339 provides a mechanical means for controlling the amount of lubricant injected into the shot sleeve. Further, neither of these methods involves the use of lubricating beads, sometimes referred to as dry lube. Solid beads are cleaner to work with than liquid and work just as well. Further, unlike solid sticks, dry lube can be injected into the shot sleeve mechanically during die casting. Hence, a means for mechanically feeding lubricating beads into the shot sleeve of a die casting machine at a controllable rate is needed.
Volumetric dry solids materials feeders are known. As noted in U.S. Pat. No. 5,301,844, various feeders have been designed "to discharge material such as powders, granules, or stranded fiberglass at a predetermined rate based on volume." U.S. Pat. No. 5,301,844 and U.S. Pat. No. 3,151,782 each disclose a volumetric materials feeder which employs a vertical feeding auger. These devices utilize a vibratory mechanism for shaking material through the auger as it turns. These devices are unsuited for dispensing dry solids which are prone to melt. Thus, a dry solids materials feeder containing a means of cooling the solids during dispensing is needed.
Horizontal screw feeder mechanisms are also known. As already noted, these mechanisms operate to extrude molten metal or some meltable substance during die casting or injection molding. See U.S. Pat. No. 4,877,568, and U.S. Pat. No. 4,044,925. These devices do not serve a volumetric feeding function, nor do they dispense dry solids. U.S. Pat. No. 5,114,648 discloses a method for pelletizing thermoplastic resin products by grinding and treating rubber compositions and extruding the resulting material through a screw feeder, They also, like the vibrating vertical auger mechanisms discussed above, have no cooling feature.
In summary, the prior art fails to disclose a method by which a beaded lubricant may be volumetrically introduced into a shot sleeve during the die casting or injection molding process. Prior art further fails to disclose a method for cooling a dry solid such as dry lube while it is being mechanically dispensed. Hence, it is an object of the present invention to provide a method for controlling the amount of lubricant placed within the shot sleeve of a die casting machine or injection molding machine. It is an additional object of this invention to provide a dispenser external to a die casting or injection molding machine which can volumetrically dispense lubricating beads without interrupting the die casting or molding process.
It is another object of the present invention to utilize a horizontal screw feeder which receives lubricating beads at one end and delivers the beads at the other, with the screw feeder operating on a timer so that the dispensing of lubricant is controlled.
It is also an object of the present invention to have a dry lube dispenser having within it an air flow operating synchronistically with the motor for assisting the beads through the screw extruder, and for cooling the dry lube pellets to prevent melting and coagulation.